It is known in the art that white or colored light illumination can be provided by an illumination system, wherein the radiation of a light emitting diode is converted by a phosphor. The light-emitting diode excites the phosphor, which thereupon emits at longer wavelength range; the combined emissions produce white or colored light.
As recent advances in light-emitting diode technology have brought very efficient light emitting diodes emitting in the near UV to blue range, today a variety of colored and white-light emitting phosphor-converted light emitting diodes are on the market, challenging traditional incandescent and fluorescent lighting.
The conventional phosphor converted light emitting device (pc-LED) typically utilizes a design in which a semiconductor chip having a blue-emitting LED thereon is covered by a layer of epoxy resin containing phosphor particles powders of one or more phosphors.
In a more recent approach, the semiconductor chip is covered by a layer of particles of one or more phosphors, which are deposited by an electrophoretic deposition technology (EPD). Such technology provides phosphor layers that are thinner than the resin-bonded phosphor layers. This allows for better chromaticity control and improved luminance.
However, a problem in prior art illumination systems comprising phosphor particle powders is that they cannot be used for many applications because they have a number of drawbacks:
Firstly, the deposition of a phosphor particle layer of uniform thickness is difficult. The phosphor particles tend to agglomerate, and hence, providing a uniform phosphor layer with particles of a known grain size is difficult. As color uniformity requires a uniform layer thickness, color uniformity is difficult to guarantee.
Secondly, conventional phosphor particles are polycrystalline powders. However, polycrystalline phosphor powder layers tend to have a low transmission due to scattering. The polycrystalline phosphor powder particles absorb scattered light that is not re-emitted, which lowers the light output. In addition, backscattering of light emitted by the LED leads to absorption losses in chip, which is relatively absorptive, leading to lower light-extraction efficiency.
WO2006/097876 discloses a light-emitting element provided with a light-emitting diode and monolithic polycrystalline ceramic structure, wherein a phosphor of the YAG-type is embedded in a ceramic matrix comprising non-luminescent polycrystalline alumina.
Embedding a luminescent compound in a ceramic matrix comprising a non-luminescent compound improves the optical characteristics of the luminescence converter and facilitates thermal management of the device.
Yet the non-luminescent polycrystalline alumina matrix material of the prior art is a material that has chemical characteristics, which are very valuable, if used in combination with a phosphor of the YAG garnet type, but may be less favorable in combination with other phosphors.
It is therefore an object of the present invention to alleviate this disadvantage and to provide an illumination system comprising a radiation source and a monolithic ceramic luminescence converter with improved characteristics.